The present invention relates to a method for recovering chemicals in a process of producing kraft pulp, wherein chemical solutions i.e. a catholyte and an anolyte, obtained by electrolyzing white liquor or green liquor i.e. an alkaline solution containing Na2S in an electrolytic cell, are efficiently utilized in a cooking step and a bleaching step in the process for producing kraft pulp, and chemicals in a liquid discharged after such utilization, are recovered, regenerated and reused.
Heretofore, as an alkali source for the oxygen delignification step in a process for producing kraft pulp, oxidized white liquor obtained by oxidizing an atomic group containing sulfur in white liquor with air in the presence of a catalyst to thiosulfuric acid, has been employed. Here, it is possible to carry out oxygen bleaching by using sodium hydroxide brought in from outside of the system. However, this means that the reagent to be used is brought in from outside of the system, and such is problematic in an attempt to proceed with a closed system of the process for producing kraft pulp. Further, the effluent from the oxygen delignification step is usually recovered by a recovery boiler, and such bringing in of sodium hydroxide tends to destroy the balance in the chemical recovery system. In order not to destroy the balance of the chemical recovery system, an alkali source derived from white liquor such as oxidized white liquor, will be required.
Besides, the above air oxidation method oxidizes sodium sulfide (Na2S) as an alkali source in white liquor to a level of sodium thiosulfate (Na2S2O3), and accordingly, the alkali source as active alkali will be made ineffective and wasted (lost). Further, in order to proceed with a closed system of the process for producing kraft pulp, also the effluent from a bleaching step subsequent to the oxygen delignification step, will be recovered by a recovery boiler, and if oxidized white liquor is to be used as a recoverable alkali source to be supplied to such a step, the required amount of white liquor correspondingly increases, and the load of the recovery boiler will correspondingly increase, such being a drawback.
Further, with respect to a method for forming a polysulfide, various methods are known, such as a method of oxidation with air in the presence of activated carbon catalyst (JP-A-47-10217), a method of oxidation with air in the presence of a lime sludge and a catalyst (JP-A-8-209573, JP-A-9-87987), a method of direct oxidation by means of an oxidation-reduction resin (JP-A-56-149304), a method of dissolving sulfur (JP-A-8-311790, JP-A-54-151602), and a method of directly forming it by electrolysis (JP-A-8-512099=PCT International Publication WO95/0071). However, one industrially practically used for the purpose of producing pulp at present, is only the air oxidation method employing the activated carbon catalyst (JP-A-47-10212, JP-A-53-92981).
However, by the air oxidation method employing the activated carbon catalyst, sodium sulfide having an effective cooking effect in the cooking step will be consumed by the oxidation to cooking-inactive sodium thiosulfate. Taking the balance of the entire white liquor into consideration, this means nullification i.e. a serious loss by the oxidation of effective sodium sulfide, and yet, an expensive oxidation installation is required for the oxidation of white liquor. Thus, the conventional process for producing kraft pulp has many drawbacks.
Further, in order to recover and reuse chemicals in the bleach effluent, the recovery boiler is required to have a reserve capacity. Loads on the recovery boiler include a load relating to organic substances and a load relating to inorganic substances, and the former can be reduced by an improvement in the yield of pulp, and the latter can be reduced by cutting back on the base unit of chemicals. Thus, a reserve capacity may be had by changing the installation or by reducing the production. However, in view of a problem in the efficiency or a problem in the cost, it is desired to rely on other methods.
As a method of improving the yield to reduce the load by organic substances i.e. the organic solid content, a polysulfide (PS) cooking method is known, and as a method for cutting back chemicals to reduce the load by inorganic substances i.e. the inorganic solid content, an AQ type assistant addition method is, for example, known. Further, as disclosed in JP-A-5-163690, JP-A-10-506687 and JP-A-10-53989, it is known to carry out better cooking by increasing the Na2S concentration at the initial stage of cooking and by controlling so that the active alkali concentration will not become lower than a certain level until completion of the cooking. Also by such a method, it is possible to reduce the load on the boiler, but by this method, it is necessary to add chemical solutions having a plurality of Na2S concentration compositions dividedly to the digester.
To form chemical solutions having a plurality of Na2S concentration compositions, it is necessary to rely on a method such as a method for thermally decomposing black liquor (JP-A-8-311790), a method for crystallizing green liquor wherein sodium carbonate is precipitated by changing the temperature or the concentration, or a method for dialyzing white liquor wherein sodium sulfide content is concentrated by electric dialysis (Journal Of Pulp and Paper Science, Vol. 23, No. 4, p.182-187, April, 1997). However, when it is attempted to combine such a method with a polysulfide cooking method, at least two steps will be required, whereby the process tends to be complex.
In addition to such various problems, a large amount of drainage is discharged in the conventional process for producing kraft pulp. From the viewpoint of the problem of environmental pollution, it is desired to minimize the amount of such drainage, even though it can not be reduced to zero. Further, heretofore, a chlorine-type bleaching agent has been used in a step of bleaching pulp obtained via a step of washing with water and a blow tank after the cooking step, but the chlorine-type substance has a problem of environmental pollution.
It is an object of the present invention to solve the various drawbacks in the conventional process for producing kraft pulp, such that (1) a material balance will be destroyed if an alkali source from outside of the system, is employed, (2) if oxidized white liquor is used, the alkali source as active alkali will be wasted, and (3) the load on the boiler increases as the range for recovery of the bleach effluent expands, and to provide a method for recovering chemicals, whereby the process for producing kraft pulp will be made to be a closed system, the yield of pulp is improved, and yet, an environmental problem is minimized.
Namely, it is an object of the present invention to solve the various problems in the above items (1) to (3) in the conventional method and to provide a method for recovering chemicals, wherein chemicals discharged from the process are efficiently reproduced and utilized in the process for producing kraft pulp, and the process is made to be a closed system whereby drainage will be made minimum or will not be discharged out of the system, whereby the process for producing kraft pulp is made efficient, and the environmental problem is solved at the same time.
Further, it is an object of the present invention to solve the various problems of the above items (1) to (3) in the conventional method and to provide a method for recovering chemicals, whereby chemicals discharged from the process are efficiently reproduced and reused without using or by minimizing a chlorine-type bleaching agent such as NaClO, ClO2 or Cl2 in the process for producing kraft pulp, and the process is made to be closed so that drainage will not be discharged out of the system or will be minimized, the process for producing kraft pulp is made efficient, and environmental problems will be solved at the same time.
The present invention is a method for recovering chemicals in a process of producing kraft pulp, characterized in that an alkaline solution containing Na2S which flows in a process of producing kraft pulp, is electrolyzed by an electrolytic oxidation method; a liquid containing a polysulfide type sulfur, which is formed at an anode side, in an amount of 6 g/l or more, is added, as it is or after being causticized, to the process before a chip has the maximum temperature, and a NaOH solution formed at a cathode side, is added to at least one step of the process after the chip has the maximum temperature till a final bleaching stage; and at least chemicals in the step wherein NaOH is added, among chemicals discharged in all steps from a cooking step till the final bleaching stage, are recovered and reused.
In the present invention, new techniques relating to kraft cooking in a process for producing kraft pulp are combined in a most suitable form, whereby chemicals discharged in the process for producing kraft pulp are efficiently reproduced, recovered and reused, and the process is closed so that chemicals will not be brought in from outside of the system, and drainage will not be discharged out of the system or minimized as far as possible.
Namely, in the present invention, with respect to the balance of material amounts in the process for producing kraft pulp, it has been found that by using an electrolytic oxidation method (which will be referred to optionally as an electrolytic method in this specification), it is possible to obtain a reserve capacity for recovery of the effluent for the recovery boiler and to form recoverable NaOH simultaneously and efficiently, and such a discovery is effectively utilized for a closed system of the process for producing kraft pulp, whereby the closed system for the process for producing kraft pulp can easily be accomplished. Further, in the present invention, a chlorine-type bleaching agent is not employed or is minimized as far as possible, whereby in addition to the above effect, it is possible to solve a problem of an environmental pollution resulting in a case where a chlorine-type bleaching agent is employed.
The present invention is constituted by combining (1) a polysulfide cooking method which is a technique to improve the yield of pulp, (2) a two stage sulfidity cooking method which is a technique corresponding to adding a chemical solution in multistages, and (3) an electrolytic method which is a technique for forming a polysulfide highly efficiently. Further, in the present invention, (4) a quinones-addition cooking method is combined therewith to obtain a further effective effect.